Mineral'^ Id Durilc 539 



fication, is the same, whether calculated from the norfti or 

 mode. 



SUMMARY. 



The minerals hypeisthene, biotite, and ihnenite occurring in 

 the dacite have been separated and analysed. The method of 

 separation was a combination of the use of an electro-magnet 

 with heavy liquids and various shaking devices, and gave satis- 

 factory results. 



These minerals and also the rock were then chemically 

 analysed, and formulae obtained for the minerals. 



The norm of the rock was calculated from the chemical 

 analysis, the mode of the rock by the application of Rosiwal's 

 method, and from this the chemical composition of the rock 

 was estimated microscopically. A close agreement was found 

 to exist between the compositions obtained chemically and 

 microscopically. 



CONCLUSIONS. 



The groundmass felspar ajipears to be a mixture of orthoclase 

 and a plagioclase (Ab., An„), and is more acid than the felspar 

 phenocrysts, as one would expect. 



In the groundmass are present all the constituents which, 

 together with either ilmenite or hypersthene are necessary to 

 form biotite, thus supporting the microscopical evidence of the 

 secondary origin of some of the biotite. 



It would also appear that the secondary biotite derived from 

 the ilmenite and groimdmass contains less magnesia and more 

 alumina than the primary biotite. 



In conclusion. I am much indebted to Professor E. W. Skeats 

 for the kindly interest he has taken in this work, and for the 

 great assistance he has rendered me whenever in difficulties, 

 also to Mr. H. J. Xrrayson, w'hose ingenuity on many occasions 

 simplitied matters considerably. 



